Illinois Attorney General Lisa Madigan announced Monday an investigation into possible wage and labor law violations by Chinese buffet-style restaurants.

Five restaurants mostly in northeastern Illinois have been subpoenaed by Madigan’s office and the Illinois Department of Labor. Madigan’s office declined to say whether the restaurants were part of the same chain.

Madigan said the probe was prompted by complaints from dozens of workers who claimed they were forced to work 11- to 13-hour shifts without breaks six days a week.

“There are a whole host of potentially illegal actions that are taking place here,” Madigan said.

Some workers, she said, claimed they were paid less than the minimum wage and were forced to sleep in garages or restaurants floors.Workers also claimed they were threatened with violence, she said.

“We are very, very concerned about this situation,” Madigan said.

Madigan urged workers with similar experiences to contact her office’s civil rights bureau at (877) 581-3692.